Many times epoxy resins, which are thermally hardened --in the course of a polyaddition reaction--with carboxylic anhydrides, amines or mercaptans, are used as reaction resins for coating, that is lacquering, covering or sheathing, or for bonding electronic components and subassemblies (Advances in Polymer Science, vol. 80 (1986), pp. 173-202). Also known are thermally hardenable epoxy resin mixtures, in which a polymerization of the epoxy resins is the basis of the hardening reaction. The polymerization can be brought about by tertiary amines or Lewis acids (C.A. May, Y. Tanaka Epoxy Resins, Marcel Dekker Inc., N.Y. 1973, pp. 283-296) or by onium salts. Thus, U.S. Pat. No. 4,216,288 discloses using an aromatic onium salt as an initiator, whereby, in addition, reductive agents can be present. From the Japanese Published Patent Application 56-152833, thermally hardenable epoxy resin compositions are known (see: Chemical Abstracts, vol. 96 (1982), no. 182205h), which contain sulphonium salts and thiophenol--as an accelerator. Also described are thermally hardenable mixtures which are storage stable and have benzylthiolanium salts (Japanese Published Patent Application 58-037003 or ChemicaI Abstracts, vol. 99 (1983), no. 141034V).
A disadvantage often encountered when epoxy resins are thermally hardened is the reduction in the viscosity of the resins in the warm-up phase before the hardening. In the case of lacquerings for example, this leads to uneven layer thicknesses and so-called lacquer runs. Also, when unhoused, bonded ICs on printed-circuit boards and hybrid circuits are covered locally by means of a reaction resin drop, this leads to an undesirable deliquescence of the drop contour. Moreover, thermal hardening processes are only conditionally suited for automated assembly lines, since the hardening process is time- and space-intensive.
In addition to the thermally activated hardening of epoxy resins, hardening reactions are also known, which are initiated by UV radiation - in the presence of photoinitiators. Effective photoinitiators are triarylsulphonium salts (see for example: U.S. Pat. Nos. 4,058,401, 4,138,255 and 4,173,476). An important characteristic of the UV initiated hardening is that the hardening reaction only takes place in the areas accessible to light. It is known that reactions initiated with triarylsulphonium salts also continue after termination of the UV radiation (see: N.S. Allen Developments in Polymer Photochemistry - 2, Applied Science Publishers Ltd., London 1981, pp. 13 and 14), however, this takes place only in the areas where the photoinitiator had been fragmented by UV radiation. However, a thermal hardening cannot be initiated with triarylsulphonium salts (see: Farbe +Lack [Dyes + Lacquer], 93rd year (1987), pp. 803-807). Therefore, UV-hardenable epoxy resin mixtures can only find limited application in the electronics field, since when electronic components and subassemblies are coated (lacquered, covered or sheathed) and bonded, areas that are shaded from light are often present.
This difficulty presents itself, for example, when lacquering printed-circuit board assemblies. The lacquers are mostly applied to the flat subassemblies by dipping them - to protect the circuit from aggressive environmental influences. A UV-hardenable lacquer, which would be advantageous for technical and economical reasons, cannot be applied in this case, because, as a result of the dipping process, the lacquer also penetrates into areas, for example gaps between the printed circuit board and components, where it is not covered by the UV radiation, and therefore can also not be hardened.
The application of UV-hardenable epoxy resins is also not possible when the UV light is so strongly absorbed or scattered in the upper layers as the result of additives, such as fillers, pigments, dyes and photoinitiators, or other resin components, that the UV intensity is no longer adequate in the deeper layers. This is the case, for example, when unhoused ICs on hybrid circuits are covered locally by means of a reaction resin drop. For technical reasons, such a reaction resin must be heavily loaded with mineral fillers and moreover have carbon black or a dye added to it. In this manner, however, the UV radiation is prevented from penetrating into the deeper areas.